Space sensors, particularly optical sensors, have thermally and optically sensitive components that must be protected from direct and often even indirect solar illumination to function. Pointing sensors (line-of-sight is steered independent of the space vehicle) require a moving sunshade over their solar exclusion angle, the minimum angle between the line-of-sight and the sensor-sun vector, to protect the sensors from solar illumination.
Several sunshade designs exist and the two most common are to either (a) connect the sunshade to the instrument being steered to wrap around the light entering the sensor regardless of where the sensor points, or (b) fix the sunshade to the space vehicle and make it wide enough and long enough to provide the necessary solar protection without blocking the light entering the sensor. These two approaches have limitations, particularly the second approach which fundamentally cannot provide an equivalent solar exclusion angle because it does not steer with the sensor. In the case of systems using a pointing “head” mirror, there is no way to move a rigid sunshade in a way that follows sensor line-of-sight without it either blocking the light reflecting off the mirror into the sensor or leaving an exposed gap between the pointing mirror and sensor at some point in its travel. Therefore, when using a pointing head mirror simply accepting the reduction in solar exclusion angle has always been the option chosen, also at the expense of a very large and heavy fixed sunshade.
FIGS. 1 and 2 illustrate one prior approach, a system 100 with a traditional tubular sunshade/baffle 102 being used to protect a gimbaled optical sensor 104. The sensor 104 is partly in an enclosure 106 that can be tilted relative to a spacecraft structure 108 in two directions. The enclosure 106 (along with the fixedly-attached tubular sunshade 102) can be rotated about both a gimbal azimuth axis 112 and a gimbal elevation axis 114. The inside of the baffle 102 is prone to heating and/or light reflection. In addition the system 100 is limited by the solar exclusion angle inherent in the baffle 102, in that the baffle 102 does not allow use of the sensor 104 when the sun 116 is within a certain angle 118 of the direction in which the baffle 102 is oriented. Increasing the size of the baffle 102 to reduce this angle can cause problems in terms of the reducing the field of view of the sensor 104 and in requiring more space and more material for the system. In addition, with reference to FIG. 2, moving the baffle 102 relative to the enclosure 106 can result in a gap 120 in the enclosure 106, which cannot be blocked with the baffle 102.
Another example of a prior solar shade for space-based sensors are those disclosed in U.S. Pat. No. 8,186,628, one embodiment of which is shown in FIG. 3. FIG. 3 shows a shade system 200 that includes an elongate curved portion 202 that extends from a base 204, and a visor 206 that is pivotally coupled to a distal end of the base 204. The base 204 may be moved azimuthally to block solar radiation at most times, with the visor 206 moved into place to block the sun when the sun is at the edge of the field of view of the sensor, such as when the sun is along the edge of a target, such as the Earth. Heating of the curved portion 202 is also a potential problem for this sort of shade.